In the formation of low firing ceramic dielectric for capacitor application, prefiring of the ceramics is traditionally required. For example, to form a single phase ceramic such as an alkaline-earth titanate-zirconate solid solution, the mix is precalcined at elevated temperature, for example, above 2200.degree. F. and is then pulverized and mixed with a glass phase and sintered at a temperature of less than 2100.degree. F.
Recently a glass reacted-ceramic dielectric has been formed from at least two distinctly different ceramic materials prior to mixing and firing to the maturity of the ceramic dielectric.
An object of this invention is to form ceramic dielectrics by direct mixing of all required ceramic compounds and sintering the same to maturity at a temperature less than 2100.degree. F. without precalcining the ceramic materials such as barium titanate and Curie point shifter or prefiring a glass material.
A further object is to provide an economic method for making a low temperature sintered ceramic dielectric material with exceptional high dielectric constant to be used for ceramic disc and multilayer capacitors and for other applications.
The process starts with the mixing of finely ground powder of commercial barium titanate plus a Curie point shifter such as SrZrO.sub.3, BaZrO.sub.3, CaZrO.sub.3, CaSnO.sub.3, and ZrO.sub.2 with a titanate flux such as PbTiO.sub.3, Bi.sub.2 Ti.sub.2 O.sub.7, and a oxide flux such as ZnO, and B.sub.2 O.sub.3 in a polymeric suspension media and milled in a ceramic ball mill to desirable particle size and distribution. The resulting ceramic slurry is cast into a film form known as ceramic tape.
Metal electrodes are deposited in paste form on the green ceramic tape. The paste consists of metal powder dispersed in a plasticized polymeric binder system with a solvent to form a screenable paste. The paste may contain 40% to 80% metal powder by weight.
The dried tape is cut and stacked in a way such that the individual capacitors are in parallel. The stack is then pressed to desirable green density to form monolithic capacitors.
The organics in the monolithics are then slowly burned off and the ceramics are sintered at an appropriate temperature to form a dense ceramic body. Metal termination materials are fired on both sides of the ceramic capacitor to form monolithic capacitors.
This invention resides in the discovery that the sintering temperature of the monolithic capacitors can be carried out below 2100.degree. F. with the addition of the flux. The low sintering temperature allows the use of more economic palladium and silver electrode paste systems.
In the drawing, the single figure shows curves of the percent change of dielectric constant plotted against temperature.